Known devices for forming an electronic image of an original by scanning the original may be generalized into four classifications, namely drum scanners, flatbed scanners, two-dimensional array scanners, and hand scanners. Drum scanners attach the original to the surface of a cylindrical drum that rotates at a substantially fixed velocity relative to an image sensor that is moved in a direction parallel to the rotational axis of the drum. Flatbed scanners typically utilize a linear sensor array that is moved relative to the original along an axis that is perpendicular to the axis of the sensor array. Similar to flatbed scanners, hand scanners include a one-dimensional array of sensors and are moved relative to the original. On the other hand a two-dimensional sensor array may be held motionless during an exposure period, if the array is sufficiently large to image the entire original during the exposure period.
Each general class of scanners offers advantages relative to the other scanners. For example, a hand scanner is easily transported. A concern with the early models of hand scanners was that the sensor array of the scanner needed to be moved in a precise path along the original in order to ensure that the electronic image did not include an unacceptable level of distortion. Thus, mechanical elements, such as cylindrical rollers or wheels, were incorporated into the hand scanners to enforce a single translational degree of freedom during the scanning process.
U.S. Pat. No. 5,578,813 to Allen et al., which is assigned to the assignee of the present invention, describes a freehand image scan device which compensates for non-linear movement of the sensor array. The scan device includes an imaging sensor and at least one navigation sensor. For example, the imaging sensor may be a one-dimensional array of sensor elements and there may be a separate two-dimensional navigation sensor array at each end of the imaging sensor. Each navigation sensor array acquires position information while the imaging sensor is capturing image information that is then “stitched” together on the basis of the position information. The navigation sensors may track movement of the imaging sensor along the original by monitoring variations of inherent structure-related properties of the original. Such inherent structure-related properties include features such as paper fibers of a paper original and specular patterns of a glossy original. Features of freehand image scan devices are also described in U.S. Pat. No. 5,644,139 to Allen et al., U.S. Pat. No. 5,825,044 to Allen et al., U.S. Pat. No. 6,005,681 to Pollard, U.S. Pat. No. 6,195,475 to Beausoleil, Jr. et al., U.S. Pat. No. 6,249,360 to Pollard et al., and U.S. Pat. No. 6,259,826 to Pollard et al., all of which are assigned to the assignee of the present invention.
A freehand image scan device of the type described in any one of the Allen et al. patents is a self-contained unit that includes an image display screen, control mechanisms (such as pushbutton controls), a power supply, memory, and image processing. The processing of captured image data and position information is computationally intensive. Image acquisition and processing is typically performed by on-board application specific integrated circuits (ASICs). The scan device may also include communication software and hardware, such as a cable connection and associated drive software or such as infrared capability.
While the currently available freehand image scan devices, as described in the Allen et al. patents, significantly extend the occasions in which document scanning is practical, further advancements are desired. What is needed is a scan capability that can be achieved in a more economical manner and that extends the capabilities of devices which are currently carried by mobile professionals.